In recent times, production of light olefins via fluid catalytic cracking (FCC) processes has been considered one of the most attractive propositions. Fuel specifications are becoming increasingly stringent due to stricter environmental regulations. Additionally, there is an ever increasing demand for petrochemical building blocks such as propylene, ethylene, and aromatics (benzene, toluene, xylenes, etc.). Further, integration of petroleum refineries with a petrochemicals complex has become a preferred option for both economic and environmental reasons. Global trends also show that there is increased demand for middle distillates (diesel) compared to that of gasoline product. Maximizing middle distillates from a typical FCC process requires operating FCC at lower reactor temperatures; it also requires utilizing different catalyst formulations. Operating at lower temperatures decreases the yield of light olefins and reduces feedstock for alkylation units.
Several fluidized bed catalytic processes have been developed over the last two decades, adapting to the changing market demands. For example, U.S. Pat. No. 7,479,218 discloses a fluidized catalytic reactor system in which a riser-reactor is divided into two sections of different radii in order to improve the selectivity for light olefins production. The first part of the riser reactor with lesser radii is employed for cracking heavy feed molecules to naphtha range. The enlarged radii portion, the second part of the riser reactor is used for further cracking of naphtha range products into light olefins such as propylene, ethylene, etc. Though the reactor system concept is fairly simple, the degree of selectivity to light olefins is limited for the following reasons: (1) the naphtha range feed streams contact partially coked or deactivated catalyst; (2) the temperature in the second part of the reaction section is much lower than the first zone because of the endothermic nature of the reaction in both sections; and (3) lack of the high activation energy required for light feed cracking as compared to that of heavy hydrocarbons.
U.S. Pat. No. 6,106,697, U.S. Pat. No. 7,128,827, and U.S. Pat. No. 7,323,099 employ two stage fluid catalytic cracking (FCC) units to allow a high degree of control for selective cracking of heavy hydrocarbons and naphtha range feed streams. In the 1st stage FCC unit, consisting of a riser reactor, stripper and regenerator for converting gas oil/heavy hydrocarbon feeds into naphtha boiling range products, in the presence of Y-type large pore zeolite catalyst. A 2nd stage FCC unit with a similar set of vessels/configuration is used for catalytic cracking of recycled naphtha streams from the 1st stage. Of course, the 2nd stage FCC unit employs a ZSM-5 type (small pore zeolite) catalyst to improve the selectivity to light olefins. Though this scheme provides a high degree of control over the feed, catalyst and operating window selection and optimization in a broad sense, the 2nd stage processing of naphtha feed produces very little coke that is insufficient to maintain the heat balance. This demands heat from external sources to have adequate temperature in the regenerator for achieving good combustion and to supply heat for feed vaporization and endothermic reaction. Usually, torch oil is burned in the 2nd stage FCC regenerator, which leads to excessive catalyst deactivation due to higher catalyst particle temperatures and hot spots.
U.S. Pat. No. 7,658,837 discloses a process and device to optimize the yields of FCC products by utilizing a part of a conventional stripper bed as a reactive stripper. Such reactive stripping concept of second reactor compromises the stripping efficiency to some extent and hence may lead to increased coke load to regenerator. The product yield and selectivity is also likely to be affected due to contact of the feed with coked or deactivated catalyst. Further, reactive stripper temperatures cannot be changed independently because the riser top temperature is directly controlled to maintain a desired set of conditions in the riser.
US2007/0205139 discloses a process to inject hydrocarbon feed through a first distributor located at the bottom section of the riser for maximizing gasoline yield. When the objective is to maximize light olefins, the feed is injected at the upper section of the riser through a similar feed distribution system with an intention to decrease the residence time of hydrocarbon vapors in the riser.
WO2010/067379 aims at increasing propylene and ethylene yields by injecting C4 and olefinic naphtha streams in the lift zone of the riser below the heavy hydrocarbon feed injection zone. These streams not only improve the light olefins yield but also act as media for catalyst transport in place of steam. This concept helps in reducing the degree of thermal deactivation of the catalyst. However, this lacks in flexibility of varying operating conditions such as temperature and WHSV in the lift zone, which are critical for cracking of such light feed steams. This is likely to result in inferior selectivity to the desired light olefins.
U.S. Pat. No. 6,869,521 discloses that contacting a feed derived from FCC product (particularly naphtha) with a catalyst in a second reactor operating in fast fluidization regime is useful for promoting hydrogen transfer reactions and also for controlling catalytic cracking reactions.
U.S. Pat. No. 7,611,622 discloses an FCC process employing dual risers for converting a C3/C4 containing feedstock to aromatics. The first and second hydrocarbon feeds are supplied to the respective 1st and 2nd risers in the presence of gallium enriched catalyst and the 2nd riser operates at higher reaction temperature than the first.
U.S. Pat. No. 5,944,982 discloses a catalytic process with dual risers for producing low sulfur and high octane gasoline. The second riser is used to process recycle the heavy naphtha and light cycle oils after hydro-treatment to maximize the gasoline yield and octane number.
US20060231461 discloses a process that maximizes production of light cycle oil (LCO) or middle distillate product and light olefins. This process employs a two reactor system where the first reactor (riser) is used for cracking gas oil feed into predominantly LCO and a second concurrent dense bed reactor is used for cracking of naphtha recycled from the first reactor. This process is limited by catalyst selectivity and lacks in the desired level of olefins in naphtha due to operation of the first reactor at substantially lower reaction temperatures.